The present invention relates to a very high temperature heating element. Heat treatments carried out at temperatures close to 2000.degree. C. are increasingly used. The furnaces and their heating elements are based on materials or ceramics which, with the aid of preheating, become electricity conductors and thus, after said initiation, constitute the resistive part indispensable for heating by the Joule effect. Such heating elements are e.g. described in French patent application No. 75 11335, filed in the name of the Commissariat a l'Energie Atomique on Apr. 11th 1975 now French Pat. No. 2,307,431.
The preheating temperatures necessary for initiating this phenomenon are approximately 1200.degree. C. .beta.-alumina (11 Al.sub.2 O.sub.3, Na.sub.2 O) permits a much lower initiating temperature of close to 200.degree. C. Thus, alumina B has Na.sup.+ ions of a much more mobile nature, whose presence therefore contributes to lowering the initiating temperature and gives alumina interesting electrical properties. Thus, an initiating temperature of close to 200.degree. C. can easily be achieved by conventional heating means, e.g. a hot air current or by using a resistor located within the ceramic heating element. However, a .beta.-alumina heating element loses Na.sup.+ ions over a period of time and as the temperature increases. Therefore its electrical properties deteriorate and the electric power and operating temperature decrease in a manner incompatible with normal usage.